TiAlN based nanoscale multilayer PVD coatings designed to...
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TiAIN based nanoscale multilayer coatings designed to adapt their tribological properties at elevated temperatures
HOVSEPIAN, Papken <http://orcid.org/0000-0002-1047-0407>, LEWIS, D. B., LUO, Q. <http://orcid.org/0000-0003-4102-2129> and MUNZ, W. D.
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HOVSEPIAN, Papken, LEWIS, D. B., LUO, Q. and MUNZ, W. D. (2005). TiAIN based nanoscale multilayer coatings designed to adapt their tribological properties at elevated temperatures. Thin solid films, 1-2.
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Sheffield Hallam University Research Archivehttp://shura.shu.ac.uk
TiAlN based nanoscale multilayer coatings designed to adapt their
tribological properties at elevated temperatures
P. Eh. Hovsepian1, D. B. Lewis
1, Q. Luo
1, W. -D. Münz
1, P. H. Mayrhofer
2, C. Mitterer
2, Z.
Zhou3, W. M. Rainforth
3
1 Materials and Engineering Research Institute, Sheffield Hallam University, Howard Street,
Sheffield, S1 1WB, UK,
2 Department of Physical Metallurgy and Materials Testing, University of Leoben, Franz-
Josef- Strasse 18, A-8700, Leoben, Austria,
3 Department of Engineering Materials, University of Sheffield, Sheffield, S1 3JD, UK
Abstract
The addition of properly selected elements, coupled in nanoscale multilayer structures can
further enhance the properties of TiAlN coatings and bring new high performance. The
incorporation of Y in the nanoscale pseudo-superlattice TiAlCrN/TiAlYN with typical period
of 1.7 nm not only improves the oxidation resistance but also effectively reduces the
coefficient of friction of the coating from 0.9 to 0.65 at temperatures in the range of 850 - 950
0C. The adaptation of the tribological properties occurs as a result of the preferential
migration of the Y to the column boundaries. TiAlN/VN superlattice can achieve another self-
adaptation process. During friction the coatings adapt themselves to the combined thermal
and mechanical wear by the formation of highly lubricious vanadium- oxides due to high
flash temperatures at the asperity contacts on the surface. The integrity of the bulk of the
coating is retained, leading to exceptionally low, for superhard coatings, friction coefficient of
0.5 and a wear coefficient of 2 10-17
m3N
-1m
-1. The coatings have been deposited by the
combined steered cathodic arc unbalanced magnetron sputtering method.
1 Introduction
The demand for high performance physical vapor deposition (PVD) coatings to resist
degradation in severe environments, endurance of extremely high thermo-mechanical loads in
particular is ever increasing. Additionally, in recent years, a demand for a new generation of
coatings capable to adapt their surface or bulk properties to these severe conditions has
emerged. In an attempt to combat wear in an environment with a variable humidity a super-
tough wear resistant WC/diamond-like-carbon/WS2 coating with “chameleon” surface
adaptation has been developed [1]. The adaptation effect lies in the synergy of smart material
and structure selection processes and has been proved for low temperatures. The combination
of nanocrystalline grains and amorphous matrix initially developed for super-hardness and
improved toughness [2 – 4] has shown also potential for self-adaptation in specific conditions.
In the medium temperature range up to 500 0C, yttrium-stabilized -zirconia/Au utilising
composite (nanocrystalline / amorphous) structures have shown consistently low friction
coefficient due to the microstructure adaptive changes and formation of lubricating Au
transfer films [5]. Recent research on the effect of the age hardening, in Ti1-xAlxN has
considered the effect as self adaptation mechanism contributing towards maintaining high
wear resistance at elevated temperatures (up to 1000 0C). It has been demonstrated that in this
temperature range, Ti1-xAlxN coatings undergo spinodal decomposition into coherent face-
centre-cubic (fcc) structured AlN and fcc-TiN nanometer-size domains. This transformation
generates an increase in hardness due to coherency stresses and therefore enhances the wear
resistance at elevated temperatures [6, 7].
The requirement for high temperature stability involves several key applications, which gain
increasing importance in modern technology, such as dry high speed machining [8],
machining of “sticky” alloys used in aerospace and automotive industries [9] as well as
protection of high duty components in aero and automotive engines manufactured from prone
to oxidation materials [10]. In all the above mentioned applications, the Ti1-xAlxN coating
family has been recognised as successful alternative, due to their excellent resistance to
micro-abrasive wear and to high temperature oxidation up to 750 0C. The remarkable high
temperature stability of these coatings has been attributed to the formation of highly
protective double layer oxide scale including a very dense Al2O3 top layer [11]. Further
enhancement of the functional properties of these coatings has been achieved by incorporating
of active elements to Ti1-xAlxN, such as Cr and Y [12]. Several mechanisms have been
proposed to explain the improved protective nature of the scale, due to the incorporation of Y,
such as improvement of the mechanical adherence of the Al2O3 scale, reduction of the scale
growth rate, grain refinement, and “plugging” diffusion paths via segregation at the grain
boundaries [13, 14].
To address specific applications however, the properties of the Ti1-xAlxN based coatings can
also be tailored successfully by structural design. Utilising the nanoscale multilayer,
superlattice structures and combining selected coating materials allows extremely high
hardness, > 40 GPa to be achieved [15, 16]. Together with the structure, the compressive
stress will also play an important role in hardness enhancement. This factor can be easily
controlled by the applied bias voltage during the coating deposition process [16]. Layering
with VN in TiAlN/VN superlattice structures in addition to the super-hardness however,
brings a unique tribological behaviour, due to the formation of a low melting point highly
lubricious V2O5, during sliding [16 – 18]. The exceptionally low coefficient of friction and
wear rates, achieved by this approach bring into consideration another adaptation mechanism,
which can be described as a compositionally regulated tribo-oxidation process, taking place
under the conditions of the tribological contact.
This paper summarises the experience gained in the development and exploitation of two
TiAlN based nanoscale multilayer structured PVD coatings namely TiAlCrN/TiAlYN and
TiAlN/VN. The ability of the coatings to self organise and adapt to the environment utilising
segregation mechanisms triggered by the exposure to high temperatures as well as by the
formation of friction reducing liquefied tribofilms of V2O5 will be described.
2 Technological aspects in deposition of nanoscale multilayer, superlattice structured
TiAlCrN/TiAlYN and TiAlN/VN coatings.
TiAlCrN/TiAlYN and TiAlN/VN nanoscale multilayer coatings have been produced in a
computer- controlled Hauzer HTC 1000-4 coater, utilising the combined cathodic
arc/unbalanced sputtering technique, known also as Arc-Bond-Sputtering (ABS) technique
[19, 20]. The coater comprises four linear dual- purpose cathodes, which can be operated
either in steered arc or unbalanced magnetron sputtering mode. Fig. 1a schematically outlines
the cross-section of the system, with a typical target arrangement used for deposition of
TiAlCrN/TiAlYN. The key advantage of the ABS approach is the utilization of a metal ion
pre-treatment (metal ion etching) of the substrate material prior to the coating deposition step,
which results in excellent coating adhesion. During the surface pre-treatment step, typically
20 min long, metal ions generated from an arc discharge sustained on one of the targets are
accelerated towards the substrate due to the applied high bias voltage (Ub = -1200 V). In the
case of Cr+ ion etching, high substrate current densities of 20 Am
-2 have been measured by
flat Langmuir probe facing the target [21]. Under these conditions, the energy of the arriving
gas-metal ions is sufficient to produce surface etching, low energy metal ion implantation and
to promote local epitaxial growth of the coating [21, 22]. Cr+ ion etching is used to treat the
surface prior to deposition of TiAlCrN/TiAlYN, whereas V+ etching is employed in the case
of TiAlN/VN coating. The macro particles emitted from the arc discharge produce growth
defects in the coating [23] without disrupting the nanoscale multilayer structure of the
coating. However, the growth defects reduce locally the oxidation resistance of the coating
due to their under-dense structure [24]. Previous work has shown that the adhesion is of
paramount importance in applications such as dry high speed milling where the tools are
subjected to interrupted reciprocating impact loads. In this application the tool lifetime is
proportional to the critical load values in scratch testing [20]. Furthermore, depending on the
deposition conditions both nanoscale multilayer coatings exhibit relatively high compressive
residual stress, up to -7 GPa, which can only be maintained by strong adhesion. This problem
is further addressed by deposition of stress reduced 0.2 – 0.3 µm thick base layer of TiAlCrN
or VN prior to the deposition of the TiAlCrN/TiAlYN or TiAlN/VN coating, respectively.
The HTC 1000-4 system is equipped with a threefold planetary rotating table, which in
combination with the multi target arrangement utilising linear sources allows straightforward
deposition of nanoscale multilayer coatings. High deposition rates (typically of 1 µm per
hour) are achieved by avoiding complicated installations, such as shielding and shuttering.
The coatings are deposited by reactive unbalanced magnetron sputtering in a common Ar +
N2 atmosphere at temperature of 450 0C. The deposition of TiAlCrN/TiAlYN coatings
requires one Cr (99.8 at.%), two Ti0.5Al0.5 and one Ti0.48Al0.48Y0.04 targets. The incorporation
of the Y in one target only is deliberate, to achieve layered distribution of Y in the coating.
This target will usually be installed between the two TiAl targets as shown in Fig. 1a. The
individual layer thickness in the nanoscale multilayer coatings is controlled precisely by the
power dissipated on the targets, sample rotation speed, applied bias voltage and the
composition of the reactive atmosphere, controlled in total pressure controlled mode (pAr +
pN2 = constant). To deposit TiAlN/VN superlattice structured coatings, schematically shown
in Fig. 1b, two pairs of adjacent Ti0.5Al0.5 and V targets (99.8% pure) have been used. All the
coatings were deposited on mirror polished M2 high speed steel, hardened to HRC 63, for
adhesion and tribological characterisation, on cemented carbides for high temperature tribo-
test, and on AISI 316 stainless steel for structural characterisation. Two-flute ball nosed
cemented carbide end mills, 8 mm in diameter, have been also coated for cutting tests. Prior
to the coating, all substrates were cleaned in an automated cleaning line, comprising a series
of ultrasonically agitated aqueous alkali solutions and deionised water baths and a vacuum
dryer.
3 Thermal behaviour of the nanoscale (pseudo-superlattice) structured
TiAlCrN/TiAlYN coating
Compared to previous generation of TiAlN coatings, TiAlCrN/TiAlYN exhibit significantly
higher resistance to oxidation and to abrasive wear. It has been reported that the onset of rapid
oxidation of TiAlN in thermo-gravimetric tests has been increased from 750 0C to 850
0C and
further to 950 0C by the incorporation in the coating of 3 at.% Cr and 2 at.% Y respectively
[25, 26]. It has been shown in [25] that the incorporation of a large and therefore relatively
immobile Y atom (atomic radius 0.18 nm) leads to repeated re-nucleation in the growing film,
resulting in a much finer grain size (typically in the range of 5 – 10) compared to that of the
TiAlN and a near equi-axed grain morphology. The incorporation of Y in TiAlCrN in a
layered manner has been found to be from a key importance for its efficiency to enhance both
the oxidation resistance as well as the high temperature tribological behavior of the coating.
The Y containing layers are separated at a distance of = 1.7 nm to form a pseudo
superlattice TiAlCrN/TiAlYN coating. The term “pseudo superlattice” is used here mainly to
emphasise the layered incorporation of the Y and the single phase structure of the coating.
Due to the small bilayer thickness however, the typical for the “true” superlattices, ( = 3 – 5
nm) hardness enhancement (the superlattice effect, [27] ) can not be realised. The typical
hardness values for this coating are in the range of HK0.025 = 27 GPa. The incorporation of Y
leads to accumulation of high residual stress, typically of -7 GPa. Therefore, in addition to the
Cr+ metal ion etching, a 300 nm thick Y free TiAlCrN base layer of low stress (-3.2 GPa) is
deposited with a main role to provide a smooth transition in stress and hardness from the
substrate material to the coating to achieve superior adhesion. This strategy proved to be
efficient as critical load values of Lc = 60 N have been measured in scratch adhesion test
when the coating is deposited on hardened high speed steel substrate [26].
To further understand the effect of Y on the thermal stability and oxidation behaviour of
TiAlN based coatings, two combinations of superlattice structured coatings were investigated,
in which TiAlYN and CrN were selected as a second partner material. It has been anticipated
that the thermal stability of the nanoscale multilayer structures will be additionally enhanced
due to the higher activation energies for diffusion and oxidation compared to the
monolithically grown structures [28]. Thermo-gravimetric analysis carried out in long
duration (up to 1000 h) iso-thermal tests in air at 750 oC showed that both TiAlCrN/TiAlYN
and TiAlN/CrN coatings are highly oxidation resistant. In this test however, the Y free
coating exhibited accelerated mass gain with increasing exposure time reaching mass gain in
the end of the test of 1.2 10-3
gcm-2
compared to 8 10-4
gcm-2
measured for the
TiAlCrN/TiAlYN coating. It has been found through detailed cross-sectional microstructure
examination that, the accelerated oxidation in the TiAlN/CrN coating was attributed to
internal oxidation of the sub-dense columnar structure. After the long term iso-thermal
process, no multilayer fringes could be seen in the TiAlN/CrN indicative of homogization of
the as-deposited TiAlN/CrN multilayer. Additionally, the coating exhibited numerous places
of inter-column porosity, Fig. 2a. The openings in the structure between the columns can be
attributed to reduction of the coating volume and development of tensile stress due to
annealing-out of voids or vacancies (Schottky defects) during the high temperature treatment
[29]. The cross-sectional transmission electron microscopy (TEM) analysis also revealed a
multitude of sites of internal oxidation in the bulk of the TiAlN/CrN coating associated
mainly with the inter-column porosity as the porous inter-column areas worked as tunnels for
the inward oxygen diffusion. In the monolithically grown TiAlCrN base layer as expected, the
internal oxidation and the structural porosity were even more pronounced and severe, Figure
2b.
In contrast, the incorporation of Y in the TiAlCrN/TiAlYN coating resulted in fully dense
columns and dense column boundaries and no evidence for internal oxidation was found. Fig.
3 shows a cross-sectional TEM micrograph of the iso-thermal treated TiAlCrN/TiAlYN
illustrating the overall structure of the coating including the surface, the bulk and the base
layer regions. On the surface of the TiAlCrN/TiAlYN coating a 1.6 m thick continuous
oxide scale had been formed followed by a transition region. The top part of the oxide scale
appeared dense, fully crystallised and rich in aluminium, which provided a barrier to the
inward diffusion of oxygen thus contributing to slower oxidation rate. In the nitride-oxide
transition region, the TiAlCrN/TiAlYN coating was decomposed by progressive release of
nitrogen followed by the formation of amorphous-like multicomponent oxide.
The higher density in aluminium- rich top oxide layer undoubtedly provides better protection
against the environmental attack. However this can not explain the high density of the bulk of
the coating retained after the long term exposure to high temperatures. This special behaviour
can be attributed to the segregation of the Y into the grain boundaries [25]. Fig. 4 shows a
scanning TEM (STEM) energy dispersive X-ray analysis (EDX) elemental map
demonstrating this unique behaviour [30]. The preferential diffusion of Y to potentially sub-
dense regions, such as grain boundaries is predetermined by the large size of the Y atom and
the kinetics of the recovery process of thermodynamically unstable phases, taking place
during annealing. It has been reported that the thermal stability of defects in the PVD coatings
strongly depends on the biaxial stress in the as deposited state [31]. The introduction of 2% Y
in the TiAlN leads to a large lattice distortion, increasing the lattice parameter ao from 0.418
nm for the Y-free coating to 0.424 nm, which results in accumulation of high compressive
stress, -7 GPa. The accumulated stress in this case, will provide the driving force in the
annealing process, whereas the recovery in the material will occur through the mechanism of
the yttrium segregation at the grain boundaries. This mechanism can be classified as a self-
healing and self- adapting mechanism. The segregation mechanism opposes and prevents
opening of large voids due to material volume reduction in the coating, by supplying Y atoms
at the vulnerable sites and therefore retaining the coating integrity. No such self-healing
mechanism operates in the Y-free TiAlCrN base layer, therefore a significant structure
changes, mainly pronounced inter-column porosity was observed, Fig. 3. The formation of
this sub- dense structure has allowed significant outward diffusion of substrate elements into
the base layer, which has been demonstrated by quantitative TEM-EDX analysis and scanning
TEM-EDX elemental mapping [10, 30]. In contrast no sign of inward and outward diffusion
of oxygen or substrate material elements has been found in the densified bulk of the coating.
Further evidence on the adaptation and self-healing effect can be found in investigating the
high temperature wear properties of TiAlCrN/TiAlYN. High-temperature wear test was
conducted on the TiAlCrN/TiAlYN and TiAlCrN coatings using a reciprocating sliding ball-
on-disk configuration, [32]. The test conditions were as follows: counterpart, 10 mm
diameter hard metal (tungsten carbide) ball, normal load 15 N (initial Hertzian contact
pressure 2.1 GPa), reciprocating stroke 2 mm, frequency 15 Hz, test duration 60 minutes, the
samples were heated to temperatures of 400, 600, 800 and 900
0C . In this test the two
coatings showed very different wear behaviour. For TiAlCrN, increasing the test temperature
from 400 0C to 900
0C led to consistent increase of the depth of the wear crater from 0.076
µm to 0.97 µm, whereas the Y-stabilised coating after an initial increase in the wear depth to
1.82 µm at 600 0C showed consistent decrease of the wear depth at higher temperatures,
reaching the lowest value of 0.82 µm at 900 0C. Further more the friction coefficient showed
similar behaviour, reducing its values from 0.9 at 600 0C to 0.65 at temperatures in the range
of 850 – 950 0C. In contrast the TiAlCrN showed consistently high coefficient of friction
increasing from 1.1 to 1.6 when the test temperature increased from 600 0C to 900
0C. This
peculiar behaviour of the TiAlCrN/TiAlYN can be related to the progressing coating
densification (self-healing) with the temperature increase due to the operation of the Y
segregation mechanism.
The superior high temperature oxidation and wear resistance of the TiAlCrN/TiAlYN coating
have also been demonstrated in a series of high-speed milling trials. Hardened to different
hardness steels and different linear cutting speeds between 385 and 500 mmin-1
have been
used to investigate the effect of the cutting temperature on the lifetime of the coated tools.
Depending on the hardness of the work piece and the cutting speed, the measured cutting
temperatures ranged from below 700 0C in cutting EN24 steel (HRC 38, cutting speed Vc =
385 mmin-1
) to 950 0C in cutting hardened A2 steel (HRC 58, Vc = 500 mmin
-1). The cutting
lifetime data of the TiAlCrN/TiAlYN and TiAlCrN coated end mills are presented in Figs. 5a
and 5b. In cutting the relatively soft steels EN24 where the cutting temperatures were below
700 0C, the TiAlCrN/TiAlYN had no advantage as compared to the Y-free coating TiAlCrN,
Fig. 5a. However, the TiAlCrN/TiAlYN coating had indeed superior performance to the
TiAlCrN in cutting harder steels and at higher cutting speeds where the cutting temperatures
were above 800 0C. In the extreme case of cutting the A2 steel at cutting speed Vc = 500
mmin-1
, Fig. 5 b, the TiAlCrN/TiAlYN doubled the lifetime of the TiAlCrN.
4. Nanoscale multilayer/superlattice structured TiAlN/VN coatings.
The excellent wear behaviour of TiAlN/VN in dry sliding conditions has been widely
reported elsewhere in [16, 33]. However, the exceptionally low wear rate of 2.1 10-17
m3N
-
1m
-1 is difficult to be explained by the superhardness (42 GPa, [16]) alone or by the distinctive
wear mechanism of the superlattice structured coatings, which suggests material removal in 6
– 8 nm thick nano layers as opposed to monolithically grown coatings, where due to severe
plastic deformation, bending and cracking of the columns, release of large 50 – 75 nm
particles is observed [34]. In the special case, when V is incorporated in the coating
constitution, the effect of tribo-oxidation wear mechanism in dry sliding has to be carefully
considered. Vanadium is known to form Magnéli-phases with oxygen, with „easy‟
crystallographic shear planes [35] that can act as solid lubricants in dry sliding conditions.
Additionally the low melting point of the V2O5 (Tm = 685 0C), which lies in the range of the
flash temperatures achieved at the asperity contacts during sliding, implies friction reduction
due to sliding over a molten phase. Cross-sectional TEM analysis of TiAlN/VN coatings of
the wear track in pin-on-disk indeed showed formation of a 10-20 nm thick tribo-film, Fig. 6.
Raman spectroscopy of the wear debris revealed the presence of V2O5 [36]. Scanning electron
microscopy (SEM) observations of the wear track after 1 million laps in room temperature
pin-on-disk test showed smooth worn surface and almost no transfer of material from the
alumina ball used as a counter part [16]. The low friction coefficient of 0.4 measured in this
test can be related to the behaviour of the tribo-oxide film containing V2O5. It is believed that
the small amount of V2O5, which forms predominantly at the asperity contacts, reduces the
coefficient of friction due to melting. The sliding wear mechanism in such conditions
resembles very much boundary lubrication, where the surfaces are separated and the junction
formation mechanism is suppressed by adsorbed molecular films (in this case molten phase)
although appreciable asperity contact may still occur. Thus the synergy between the fine
delamination wear mechanism characteristic of superlattice structures and the low melting
point of the tribo- film controls the low friction coefficient at room temperature. In the view
of potential high temperature applications the oxidation and the tribological behaviour of
TiAlN/VN at elevated temperatures have been studied extensively [17, 18, 37]. A
combination of differential scanning calorimetry investigations and high temperature (up to
700 0C) tribometry revealed a unique self- adapting behaviour of TiAlN/VN to combined
thermal mechanical and chemical wear, [17]. It has been shown that at 700 0C in steady state
conditions, low friction coefficient in the range of 0.5 was achieved, Fig. 7, which in effect is
similar to the friction coefficient measured at room temperature. To explain this effect an
operation of a specific controlling mechanism has been suggested that involves sliding against
liquefied oxide scale and conversion of V2O5 to lower oxidised Magnéli-phases [17]. The
very low value for the coefficient of friction of 0.18 (Fig. 7), measured for the initial stages of
the sliding process was related to a sliding over liquefied V2O5 phase. A SEM cross section,
Fig. 8a, produced through the wear track in TiAlN/VN evidences conclusively the existence
of a molten phase during sliding at 700 0C. A smooth surface, with typical features for a
solidified layer can be observed. The more detailed view shown in Fig. 8 b, indicates that the
melting occurs just for the near- surface region, which also implies that the bulk of the
TiAlN/VN coating remains intact at these temperatures. The second part of the friction curve
in Fig. 7 represents the friction behaviour in steady state conditions. The exceptionally low
friction coefficient of 0.55 for hard coatings at this temperature range is believed to result
from sliding against low shear strength Magnéli-phases of V6O13 and VO2 produced by a
reduction process of V2O5 [38]. The operation of the above- described mechanism however,
requires availability of significant amount of V2O5 phase, the largest portion of which at 700
0C is produced by thermal oxidation process. Thermo-gravimetric oxidation experiments and
isothermal oxidation tests at 550 0C for 30 min showed, that the surface of the TiAlN/VN was
almost free of V2O5 [18, 37]. Fig. 9a shows an SEM image of the surface after isothermal
oxidation at 550 0C. After 30 min exposure, only very thin oxide layer with a needle like
morphology has been formed, which did not produce intensive enough signal to be detected
by X-ray diffraction. In the temperature range of 500 0C indeed significant increase of the
friction coefficient was recorded [17], which implies for the existence of a transition regime in
the control mechanisms operating at room and elevated temperatures. At higher temperatures,
above 600 0C however, formation of V2O5 mixed predominantly with AlVO4 is detected in
sufficient amount to trigger the control mechanism for low friction in steady state conditions.
An SEM image of the surface after exposure for 30 min to 638 0C shows that the coating
surface was uniformly oxidised, Fig. 9b. A large area of the surface was covered by fine
needles/plates, which form with a certain preferred orientation (arrowed). EDX selected area
analysis showed that these fine needles/plates were highly V-rich. Further X-ray diffraction
analysis confirmed the presence of the V2O5 phase. Fig. 10 shows a bright field cross-
sectional TEM image of oxide / coating interface region from sample isothermally oxidised at
600 0C for 30 min. The cross section clearly demonstrates that the high temperature did not
affect the superlattice structure of the TiAlN/VN below the top oxide layer. The Fresnel
contrast from the nanoscale multilayer structure extended right up to the oxide / coating
interface, suggesting that no inter- diffusion between the individual layers has taken place.
This observation demonstrates that the bulk of the coating retains the enhanced mechanical
properties of the superlattice coatings and the material responds to the environment by
adapting its surface properties only. The consistently low and almost identical coefficient of
friction recorded for room and 700 0C determine the major application fields for TiAlN/VN,
such as protection of wear parts and cutting tools used for machining of softer but much
“stickier” materials e.g. Ni based alloys. TiAlN/VN coated 8 mm two flute ball nosed end
mills were tested in dry cutting of Inconel 718, HRC43, cutting speed 90 mmin-1
, axial depth
of cut 0.5 mm, feed 0.2 mm. The superlattice coated tools showed the longest length of cut of
380 m, out-performing various PVD coatings: TiAlN, TiAlN+WC/C, TiAlN+MoS2,
TiAlN+graphite, thus demonstrating the importance of maintaining the low coefficient of
friction at elevated temperatures.
5. Conclussions
The experince gained so far in the design, large scale manufacturing and application of
nanoscale multilayer/superlattice structured PVD coatings allows us to identify two TiAlN
based coatings adapting their tribological proparties at high temperatures. In the
TiAlCrN/TiAlYN system, high wear resistance and low friction coefficient are achieved due
to a segregation mechanism taking place in the bulk of the coating, which results in coating
structural densification at high (800 – 950 0C) temperatures. In the TiAlN/VN system,
exceptionally low coefficient of friction and wear rates are acieved by the adaptation of the
surface properties, due to a compositionally regulated tribo-oxidation process taking place
during sliding.
Acknowledgements
This paper has been written as a survey article. The great support of all the Surface
Engineering Research Group at the Materials and Engineering Research Institute at Sheffield
Hallam University is acknowledged. Parts of the work have been carried out within DTI-Link
Surface Engineering Programme GK/K76351 MULTICOAT and UK Engineering and
Physical Science Research Counsil (EPSRC) Programme, grant No. GR/N23998/01.
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List of Figure Captions:
Figure 1 Schematic cross sections of (a) the four cathode HTC 1000-4 PVD coating system,
and (b) schematic of a typical superlattice structured coating design.
Figure 2 Cross-sectional TEM micrographs of (a) bulk of the TiAlN/CrN superlattice coating.
and (b) the base layer region after iso-thermal treatment (T = 750 0C, for 1000 h).
Figure 3 Cross-sectional TEM micrograph of TiAlCrN/TiAlYN coating, iso-thermally treated
at T = 750 0C, for 1000 h.
Figure 4 STEM-EDX maps (a) L-Y and (b) K-Y of TiAlCrN/TiAlYN coating, showing
segregation of Y at the grain boundaries.
Figure 5 Life-time of 8 mm in diameter two flute ball nosed end mills coated with various
coatings (a) in cutting soft EN24 steel (HRC 38) and (b) in cutting hard A2 steel (HRC 58).
Figure 6 Cross-sectional TEM micrograph of TiAlN/VN superlattice coating from the wear
track region after pin-on-disk test showing the tribo-film formation.
Figure 7 Friction coefficient curve of TiAlN/VN superlattice coating tested at high
temperature of 700 0C in air atmosphere.
Figure 8 (a) Cross-sectional SEM overview of the wear track on a TiAlN/VN superlattice
coating after a tribo-test at 700 0C, (b) Detailed image of the oxidised coating surface from the
indicated area in Fig. 8 a of the wear track.
Figure 9 (a) SEM image from the surface of TiAlN/VN superlattice coating iso-thermally
treated at T = 550 0C, for 30 minutes; (b) after oxidation in air at 638
0C for 30 min.
Figure 10 Cross-sectional TEM bright field image of oxide/coating interface region of
TiAlN/VN superlattice coating oxidised in air at 600 0C for 30 min.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10